1. Field of the Invention
The invention relates in general to a method of fabricating a solder mask and a structure of a substrate. More particularly, the invention relates to a structure of a substrate enduring relatively poor surroundings and a method of fabricating a solder mask of the substrate.
2. Description of the Related Art
Recently, following the change of electronics technology with each passing day, high-tech electronic products with relatively comfort and multi-function have been presented to the public one after another. The nucleus of the electronic products is chips electrically connecting with other chips or passive units through a substrate. Generally, spread on a surface of the substrate is a solder mask that can prevent solder from extending to neighboring conductive pads during a reflow process and, therefore, can avoid short circuits. The method of fabricating the solder mask is described as follows.
FIG. 1 and FIG. 2 are schematic cross-sectional views showing a process of forming a conventional solder mask of a substrate. Referring to FIG. 1, After a outermost metal layer 910 is formed, a solder mask 920 is formed on a surface 902 of the substrate 900 using a spin-coating method and the solder mask 920 covers the metal layer 910. The material of the solder mask 920 is, for example, a photoimageable dielectric. Subsequently, using an exposure process and a developing process, a pattern can be transferred to the solder mask 920 so that many openings 922 are formed through the solder mask 920 and expose the metal layer 910, as shown in FIG. 2. Finally, a curing process is used to harden the solder mask 920. The manufacture of the solder mask 920 is finished so far. The metal layer 910 exposed through the openings 922 of the solder mask 920 is defined as conductive pads 912 by which the substrate 900 can be electrically connected with outer circuits.
In the above process, generally, the dielectric material inside the substrate 900 is bismaleimide-triazine (BT) or epoxy resin that is different from the material of the solder mask 920. Moreover, the glass transition temperature of the solder mask 920 is even low. Therefore, the substrate cannot be applied under poor conditions, for example, under lead-free reflow conditions where the temperature should be raised up to over 200 degree. C. or under a moisture sensitive condition I or II (MSC I or MSC II). When the substrate 900 is under poor surroundings, the expansion stress of the solder mask 920 can be so high that the solder mask 920 can be likely separated from a molding compound or from the inner material of the substrate 900. Consequently, the solder mask 920 is the most fragile portion in a package structure.